text:
Is it normal for visibility to be missing from the METAR?
I have noticed at a local airport the AWOS METAR oftentimes is missing visibility measurements. In talking with other more experienced pilots, they tell me this isn't an isolated thing. They indicated that they read between the lines of the other data on the report (or other reports) to get an idea of what weather to expect.
Example:
KTPF 132055Z AUTO 29004KT SCT080 30/22 A2991 RMK AO2 PWINO
The recording always states "VISIBILITY MISSING" meaning it has a sensor and is just... not working?
My questions are:

Is it really normal to see reports without viz?
Why does this happen so often for AWOS when ATIS doesn't have problems with it?
Should this be reported to some entity (NOAA/FAA) to get these systems repaired or exchanged?
The fact that it is missing visibility information does not mean that visibility is unobstructed, right? I have heard people assume this.
If the recording says "VISIBILITY MISSING", is it indicated in the METAR somewhere?

PWINO indicates the present weather indicator is INOP.  Not sure if visibility is rolled into that or not.

My questions are:
Is it really normal to see reports without viz?

Yes!  Well, maybe not "normal", but it does happen.  Automated weather stations are like another piece of equipment and components fail from time to time, including the components which measure the visibility.  The other components also fail from time to time (I've see ASOS/AWOS reports without temperature, barometric pressure, etc.)

Why does this happen so often for AWOS when ATIS doesn't have problems
  with it?

You shouldn't see this at all with ATIS, because a human is involved, and if the equipment which measures visibility is inoperative, a trained observer (often the same person that is manually recording the ATIS) can look out the window and record the visibility themselves.

Should this be reported to some entity (NOAA/FAA) to get these systems
  repaired or exchanged?

I would let the airport manager know just in case they weren't aware.  They are responsible for notifying the FAA.

The fact that it is missing visibility information does not mean that
  visibility is unobstructed, right? I have heard people assume this.

If the visibility is missing, it means just that.  It is missing.  It might be 1/4 SM or it might be 20 SM. Some people may make assumptions that if there is no precipitation, that the temperature / dew point spread is high, etc. that the visibility is good, but that is a potentially dangerous assumption because visibility can be influenced by many different things.  There could even be a dust storm coming through when everything else looks good!

If the recording says "VISIBILITY MISSING", is it indicated in the metar
  somewhere?

Sort of.  It's just "missing" (i.e. the entire wind block from the METAR will not be there), so if you don't see it then, well, it's missing.

Thanks for your answer! I have also noticed the wind measurements are oftentimes way off for automated stations. Is this just a fact of life with AWOS systems or am I just encountering crappy sensors!?

@Charlie Well, sometimes it depends on the installation (I have seen stations that are never accurate if the wind is coming from the West where the trees are, etc.), and sometimes the winds are variable so it is hard to be accurate.  Other times they are just crappy, lol.


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How much force does a 747 land with?
I've been told before that the runways for large commercial aircraft have to be built to withstand a large amount of force.  But I'm wondering exactly how much it is?  When a 747 touches down, how much force is being imparted from the wheels to the tarmac?  And, further, how much force is imparted into the runway while the plan is braking right after touchdown?
Bonus Points:

How much weight does the gear + the wheels absorb and dissipate?
I'm sure there's a range from a "greaser" landing with light braking all the way to whatever the highest the main gear is rated for, it would be nice to know both extremes and the mean.

Here's an interesting document about the forces on a 747 landing gear, but note a lot of the sources cited within have broken links, so I've not added as an answer. http://www.freewebs.com/josfrijmann/files/pro/Report%205%20Landing%20Gear.pdf

Also, I think a single strut should be able to take the weight / impact from the whole plane, since that may just be the strut to hit first, so the runway in turn should be able to take the impact of a single strut.

Yes, certifications require single strut landing (in case of cross-winds, etc).

I do not know the exact numbers for oleo stroke and so on, but this is how you would calculate this. A 747-400 weighs 400 tons on take-off and 296 tons on landing, maximum. See here for the source of those figures.
Next is the landing speed, this is approx. 160 knots = 82 m/s. Now let's assume the pilot has misjudged the height and does not flare, but slams the aircraft with the 3° approach angle into the runway. It is supposed to survive this, so let's just continue. This gives you a vertical speed of 4.3 m/s, and at 296 tons this is an energy of 2,750 kWs = 0.76 kWh, which needs to be dissipated by the landing gear. Now I make the assumption that the gear stroke is 0.5 m (those who know please put it in the comments, and I will correct the calculation). We have 0.5 m to decelerate a mass of 296 tons from 4.3 m/s to zero. If we assume a constant deceleration, the force will also be constant and the sink speed will decrease linearly.
The average sink speed during this process is 2.15 m/s, so it will take 0.23 s and an acceleration of 18.5 m/s$^2$. Force is mass times acceleration, so the force is 5,473 kN or 1.23 million pounds. This is just the inertial force to stop the descent. While taxiing to the take-off position, the aircraft will press with 400 tons = 878,000 lbs on the runway, since the wings do not yet produce any lift. This shows that even a hard landing does not stress the landing gear so much - after all, the acceleration is just shy of 2 g, acting on a much lighter plane.
In reality, the landing gear of a 747 is staggered, so the inner main gears will touch the ground first. Also, I expect that the force will not be constant along the whole gear stroke. This will change the details of this approximation, but the general magnitude should not be different.
This answer goes into more details on how to calculate the damage an airplane will do to a given runway or apron.
Now for the braking stresses. The landing field length of a 747-400 is 2175 m, and let's just assume that the pilot forgot to use thrust reversers, aerodynamic drag was switched off that day, and all the braking power had to be supplied by the 16 main wheels. Let's also assume that the pilot uses 1200 m of that field length for braking (I'm just making this up to get to an upper limit of what force will act on the tarmac). Now we need to decelerate from 82 m/s to zero within 1200 m. Linear deceleration means an average speed of 41 m/s, so the whole process takes 29.27 s. Dividing speed by time yields a deceleration of 2.8 m/s$^2$.
To bring the aircraft to a full stop, we have an energy of 995,152 kWs = 276.4 kWh to dissipate along a distance of 1200 m. Using again Newton's second law, we see that this requires a horizontal force of 829 kN = 186,322 lbs, which translates into 51,8 kN = 11,645 lbs per wheel. This is certainly more than what happens in reality, but to put it into proportion: The static load per main wheel at the maximum landing mass is 174 kN = 39,150 lbs (assuming that 4% of the mass is carried by the nose gear). This extreme (horizontal) braking force is still less than 30% of the (vertical) static load, which is well below of the maximum braking coefficient of an aircraft wheel on a dry runway.
EDIT:
CGCampell correctly remarked that emergency procedures at take-off weight will produce the biggest braking loads. Now I will calculate the highest possible braking loads, and for this I need this plot of a polynominal for the braking coefficient, which is the ratio between vertical and horizontal forces before the tire skids. I do not know the source; I collected it somewhere in the past and never found a reason to doubt its validity.

Shortly before the aircraft comes to a stop, the highest friction coefficient is reached, and then little lift is produced by the wings, so the vertical tire loads are those of the static case. At 96% of 400 tons acting on 16 wheels, this is 24 tons = 235,344 N = 52,907 lbs of downward force per wheel. Since the friction coefficient is 1 at low speed, the same load is transferred horizontally from each wheel to the ground, almost five times more than what I approximated for the landing above. Clearly, going to the limits produces much higher loads.

Formulas used in the calculation:
How to calculate the deceleration of a mass $m$ from a speed $v_0$ to $v_1$
Energy has no absolute value, we just add or subtract energy. The amount of energy change $∆E$ between the aircraft at a speed $v_0$ and at a speed $v_1$ is:
$$∆E = \frac{m}{2}\cdot (v_0^2 - v_1^2)$$
The unit of energy is the Joule, which equals the Watt-second or the Newton-meter. Thus 1 J = 1 Ws = 1 Nm = 1 $\frac{\text{kg}\cdot \text{m}^2}{\text{s}^2}$. Metric units are neat, right?
In physics, energy $E$ equals work $W$, and work is performed when a force $F$ travels along a distance $s$. As does the braking force, acting on the moving aircraft:
$$E = W = F_{brake}\cdot s$$
How do we get the rollout distance $s$?
We use the fact that the energy change due to deceleration equals the brake energy:
$$\frac{m}{2}\cdot (v_0^2 - v_1^2) = F_{brake}\cdot s \Leftrightarrow s =  \frac{m}{2\cdot F_{brake}}\cdot (v_0^2 - v_1^2)$$
How can we find the time $t$ it takes to slow the aircraft down?
If the force is constant, the acceleration $a = \frac{m}{F}$ is constant, too. So the speed over time $v(t)$ is:
$$v(t) = v_0 + a\cdot t$$
We decelerate, so $a$ is a negative number. But the same works for a positive acceleration, too. To find the time $∆t = t_1 - t_0$ we make use of the fact that v varies linearly over time, such that the average speed is the arithmetic mean between $v_0$ and $v_1$. And time is just distance divided by speed:
$$∆t = \frac{2\cdot s}{v_0 + v_1} = \frac{m\cdot s}{F_{brake}\cdot(v_0 + v_1)}\cdot (v_0^2 - v_1^2)$$
With such formulas it is always a good idea to compare the units on both sides. Yup, seconds, so the result is really a time.

Wouldn't you use takeoff weight, since an emergency could required return to field at max takeoff weight (less fuel used in return).

@CGCampbell on the aircraft in question you would dump fuel to get to the proper landing weight...

I know this question is 747, and the video here is a 757, but "ThomsonFly 757 bird strike & flames captured on video", http://youtu.be/9KhZwsYtNDE this plane returns to the airport with a full load.

@CGCampbell Ah, good point, I keep forgetting sometimes procedures have to be put off in the face of an emergency.  Apologies.  The original question was supposed to be about ranges of normal operation though, I probably should have just started by saying that.

@CGCampbell: Good point! An aborted take-off should produce the biggest braking forces. More load on the wheels means more braking power. I could also use actual friction coefficients for rubber and concrete to get to the maximum load possible. I will edit the post accordingly.

Hard landing reports on AvHerald often include the peek vertical acceleration and many list value as low as 1.8G. So I'd say normal operation is probably up to 15 m/s². On the other hand not flaring will probably yield peek vertical acceleration over 20 m/s² (2G).

For the braking force, coefficient of friction of rubber on concrete is about 0.8 and cars usually have deceleration ~0.65G on dry asphalt. Aircraft will have slightly less due to cooling issues. So I'd just take 0.6 times weight for the maximal braking force. Usual rate will be much less; I believe medium autobrake setting is usually around 7 ft/s² ≃ 0.22 G.

Doesn't the landing  surface deform as well?

@rbp: Yes, but this is really negligible. Compare the stiffness of several feet of reinforced concrete with that of the tire - oleo strut combination, and it should be clear that most of the deformation is on the side of the landing gear.

I didn't follow quite everything on the landing and aborted takeoff scenarios, but wouldn't most of the force be dissipated as heat via the brakes?

@Lnafziger not the force, but the energy. The landing impact is absorbed by the oil in the oleo legs which heat up (and then dissipate this heat slowly), and the braking energy is absorbed by heating the brakes and by rubbing of a little rubber.

One random thought, could this massive energy be tapped or converted to another form of re-usable enerygy, either by the aircraft or through the runway?

@Firee: Hub motors are being tested now, and they could be run as free frequency generators, acting like a weak brake. However, the amount of energy that can be absorbed this way is rather small, so most of it will have to end up heating the brake discs. Also, the oil in the oleo strut could run trough a small turbine instead of a narrow slot, and again the energy off take would need to be tailored to the requirements and only a fraction can be used. Generally yes, but it sounds rather complicated and will not be very efficient.

Right, I was asking about the aborted takeoff.  Since the question was asking how much force was generated in the pavement and it looks like you are calculating the energy required to stop the aircraft (which would be very different because of the heat that you describe).

@Lnafziger I don't understand your comment. What has heat to do with kinetic energy? I added an explanation of how to calculate the parameters; maybe this is more helpful.

Okay, I read the question as basically "how hard does the airplane hit the ground when it lands", and specifically asks "how much force is being imparted from the wheels  to the tarmac."  I don't understand how the part about calculating the amount of energy required to stop the airplane, using the brakes, answers this question.

@Lnafziger Ah - now I get it. The part about hitting the tarmac deals with vertical forces, while the rest deals with lateral forces, which damage the tarmac (or, better, concrete), too. Both are forces occurring during the landing process and are imparted from the wheels to the ground, so I thought it is best to cover both of them. You should be used by now to my  tendency to cover more than one aspect only ;-)

Right, are you able to calculate how much force is actually  transferred to the tarnac/concrete during an aborted takeoff versus how much is dissipated as heat?  I would think that the actual transfer of energy would be much lower than the scenario where the aircraft touched down from flight and then decelerated.

The aborted take-off uses a much heavier aircraft, and when deceleration only starts at $v_1$, the whole process is very similar to a regular landing, but with a heavier aircraft. Regarding heat: Practically all the kinetic energy is transformed to heat, mostly by friction (air - surface, rubber - concrete, brake pad - brake disc are just the most important pairings). The forces do not produce heat by themselves, and there is no split (some energy goes into forces, other into heating) - that is wrong and does not happen! Only the force acting along a distance creates work and causes friction.

I just saw this question and wanted to add my perspective from the point of view of a structural engineer who flies for fun as a private pilot.
I think the whole energy of impact of the airplane has to be dealt with as a non-elastic collision between the plane and the concrete slab of the runway! Assuming we can ignore the damping effect of landing wheels!
The plane hits the runway and  becomes one with it and starts to press and drag it down with the total mass of the plane and the mass of concrete slab ( part of it which is affected by impact) and a trapezoid influence area of crushed and compacted aggregate underlayment. In the absence of structural data on the properties of runway slab it is not possible to come up with even a rough numerical estimate of how much the flexibility and dynamic response of slab helps. A vibrating trail of the slab will follow the landing gear lie a long tidal wave that is powered by the impact of the tires and takes away from the shock of impact.
This means the deceleration length is more than 0.5 meters, The kinetic energy of landing is dissipated by a large amount by slab and its substructure. So the impact is softer.
As far as braking forces the undesired heating of tires on touch down smoking has an incidental benefit of raising the coefficient of friction and helping with the braking.
As a side note, piloting small lightweight planes is fun and has many educational benefits too. The fact that brakes are not automatic allows you to burn them in wet runways a bit for better grip!

Why on earth would you neglect the landing wheels' influence? Especially for a 747 which has large stroke suspension. Really, the more logical choice would be to neglect the runway properties and assume it's a rigid body (even when assuming rigid suspension, the deformation of a tyre is a magnitude more than the runway deformation)

I meant ignoring tires' pneumatic deflection only, not landing gear! I said the stroke would be greater even.  If you want to be a bit more careful you have to consider the fact that the wings are designed to be very flexible and in a hard landing they bend and gently deflect breaking the impact. In fact the entire fuselage and empennage is designed to flex longitudinally like a dolphin.  So much so that passengers at the rear complain of vibration.  Analysis of  landing should consider the dynamic interaction of plain, air lift, and runway, being mindful even of the harmonics resonances.

Still - do you think the displacement of the tarmac will be significant compared to the 0.5m stroke? By completely ignoring the tarmac, any calculation of the runway-tyre force will be off by ~10% tops. Of course, the stresses may be a lot more than you would expect based on the forces, but that's not what the question asks.

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Is ground school required to get a Private Pilots License?
I know that there is a written exam that you are required to take, but do you have to go to a class in order to study for that test?  If I'm really good at just learning from a book, can I just learn that way, maybe take a couple practice tests and then take the exam?  Or are they going to need proof that I went to some certified class?
More subjectively: Assuming it's allowable, is it a bad idea?

When it comes to regulatory questions, make sure to let us know what country you're in!

Added that in :)

My students have not been typical, and many are scientists and engineers, and motivated high school students.  Only one, out of hundreds has taken a formal "ground school."  Probably 50 (just a guess) have bought, borrowed or somehow used videos or online coursework.  The rest (hundreds), have elected to study on their own.  I provide a modified FAA syllabus, my supplemental syllabus, and pointers to a large number of resources.  I also provide a reading list.  Since I am in the Northeastern US, they read "Weather Flying," by Robert Buck.  Even if they are a primary student, I want them to understand many of the things Buck talks about.  But that is just an example.
Most of my students meet with each other for review sessions, and if they get stuck on something, I might get a call.
This method won't work for everyone.  Many flight schools make money by offering ground schools.  Many CFIs are more comfortable with their students getting a comprehensive ground course in a more formal setting. Some students might be more comfortable with a structured classroom.
However, when the students have free reign to explore their material, I find they do better.  As an example, one student had a hard time with turbulence in flight as a primary student, and she studied weather phenomena and became quite knowledgeable on lifted index, and so on. This is typical of students who are curious and motivated.
So if one is disinclined to go with a traditional ground school, and your instructor(s) are supportive, from my experience it is not only doable, but can produce superior results.  Oh, on the written?  I cannot remember any student getting below a 94.  It may have happened, but if it did it was unmemorable.
I realize that this answer is not laden with regs and manual quotes, but it reflects experiences having instructed for 40 some years.
An aside for CFIs, the rules may have changed recently, but I normally sign off for the written with my AGI certificate.  The record keeping requirement is different.

A ‘ground school’ per se is not a required part of a Part 61 flight training program but ground instruction is an integral component of Part 61 instruction per §61.105 and §61.107.  The FAA has tightened down considerably on this stuff and it’s a good idea for CFIs to document this ground work with logbook endorsements verifying all items of §61.105(b)(1-13) and §61.107(b)(1-10).
As for home study ground school programs like Sportys, King Schools, Jeppesen, etc., they are acceptable, though I recommend using the FAA handbooks for this as they are much more concise and succinct and the FAA will inevitably test you on their content.  If you have questions from the reading, you are advised to discuss this with your flight instructor.  As to in classroom ground schools, I can advise that they are useful in some respects as they can help you with ‘catches’ in the regulations and learning the enormous amount of material that goes into flight training.

As noted above, a ground school is not required by the FAA but a sign-off by an instructor is.
I do not know if this is still valid, but a decade ago, one of the big pilot training companies (King? Sporty's? Gleim?) would send you the equivalent of an instructor's endorsement after you took that company's ground school course in the comfort of your own home.  So at least back then, yes, if you sent in the required completed home school tests, you did not need to attend a ground school class.
Is it a good idea?  I vote NO.  the reason is that these courses teach pretty specifically to the tests and little else.  There is a lot of knowledge that you won't get by taking these courses without an instructor even though you can pass the test.  Remember that they are teaching the MINIMUM you need to be safe.  Choose well.

I used the Sporty's iPad app and it has a feature which sent you necessary sign off (which you print out) once you had scored a minimum of, I believe, 85 on at least three practice tests. I presented mine when I took my written and it was accepted without question.

As of 2017-Jul, Gleim provides a sign-off to take the written test after you have correctly answered every question at least once.

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Why do jet engines get better fuel efficiency at high altitudes?
I'm told that this is true, but I can't imagine why.  It seems like the fact that there is less air would make the engines less efficient...  But that probably just shows how little I know about jet engines.

On a slightly different note, the Concorde was super-efficient at its cruise altitudes when flying at Mach 2.00+. The same can't be said about its low altitude subsonic efficiency :)

Less air makes the engines less powerful. But efficiency is ratio of power generated to fuel consumed .

Probably because designs require trade-offs and the engineers chose to optimize the engine for high altitude operation since that's where it spends most of the time.

For a quick explanation, you need to know that

Thrust is the difference between the entry impulse of the air entering the engine and the exit impulse of the heated fuel-air mixture leaving the engine. Impulse is mass times velocity, and expressed with a mass flow $\dot m$, thrust T is $$T = \dot m \cdot (v_{exit} - v_{entry})$$
The exit impulse is increased by accelerating the airflow through the engine, and the acceleration is achieved by heating the air.
Each gram of fuel heats up a given mass of air by a certain number of centigrades. The definition of the energy content of fuels is given as the capacity to heat one pound of water by one degree Fahrenheit. The definition of one Calorie is similar but in metric units. Since the heat capacity of both water and air are almost constant at moderate temperatures, the starting temperature makes little difference to the absolute temperature increase when a given amount of energy is added.

Thermal efficiency
Thermal efficiency is the ratio between the mechanical work extracted as thrust and the heat energy spent on heating the air, and it is affected indirectly by the flight altitude. Please see the Wikipedia article on the Carnot cycle. This and similar cycles describe the workings of all combustion engines in thermodynamic terms. Basically, it says that the efficiency of a combustion engine cannot be greater than the temperature ratio between the temperature increase from ambient ($t_{amb}$) to the maximum temperature $t_{max}$ of the process, divided by the maximum temperature. All temperatures must be expressed as total temperatures, where 0° means 0 K or -273.15°C. Operating in colder air makes the ratio bigger and improves efficiency.
$$\eta_t = \frac{t_{max} - t_{amb}}{t_{max}} $$

If $t_{amb}$ is 290 K (16.85°C or 62°F) and the fuel heats up the air to 1400 K (2060°F), the thermal efficiency according to the formula above is 79.3%.
At cruise altitude $t_{amb}$ is only 220 K (-53.15°C or -63.7°F), and the same fuel flow relative to air flow will lift the maximum temperature only to 1320 K (in reality even less; for more precise reasoning see below). Now the thermal efficiency is 83.33%! If the maximum temperature is maintained, both thrust and thermal efficiency will go up; the latter to 84.3%.
In reality, the total efficiency will be lower because we have not included propulsive efficiency, friction effects or power offtake by bleed air, pumps and generators. Propulsive efficiency describes how well the acceleration of air is performed.
Heating the fuel-air mixture
Burning a fuel-air mixture will add thermal energy to it, about 43 MJ for every kilogram of kerosene (if we assume complete combustion). The isobaric heat capacity or specific heat of air (close enough, the mixture has very little fuel but lots of air in it) is 29 J per mol and per K, so those 43 MJ will heat 1000 mol of air by 1483 K. The heat capacity changes slightly with humidity and temperature, but little enough that we can consider it constant for this purpose. If the air starts at 220 K, precompression in the intake will heat it to approx. 232 K, further compression in the engine will heat it up to approx. 600 K if we assume a compression ratio of 25, and this is the temperature at the entry of the combustion chamber.
Those 1000 mol of air weigh about 29 kg, and adding a full kilo of fuel and burning the mixture will heat it to 2083 K. If you want more details about the parameters in a typical jet engine, please see the diagram in this answer. Since the mixture picks up speed as it burns, the fuel mass is also heated and combustion is never complete, the maximum temperature given here will not be reached in reality.
If we start on the ground with an air temperature of 290 K, the temperature in the intake would drop slightly because we will not be flying fast enough for any precompression to happen in the intake. Now the compressor will heat the air to 730 K, and again adding and burning that kilo of kerosene will heat 1000 mol of air to 2213 K. Ideally.
In reality, the engine control will see that the limit temperatures are not exceeded, but here we can play with the numbers as we like. The exact values will certainly be slightly different (more frictional heating in the compressor, heat loss to the outside, slight drift in specific heat with temperature), but the gist of the explanation is correct.
Explanation in layman's terms
Burning the fuel-air mixture heats it and makes the gas expand. This happens at nearly constant pressure and in a duct with a given cross-section, so the only way to make room for this expansion is for the gas to flow faster. Nearly constant pressure means that the density of the gas must decrease. The density ratio between the heated and the unburnt gas is proportional to its temperature ratio, measured in absolute temperature.
However, the amount of fuel burnt determines the absolute temperature increase, the difference in degrees between the burnt gas inside the combustion chamber and the unburnt gas at the intake. For a given amount of fuel, the temperature ratio which can be achieved with an absolute temperature increase becomes smaller the higher the temperature of the unburnt gas is. Thus, efficiency decreases with a higher temperature of the intake air.

Wow, great expansion!  I know this is asking a bit, but could you include a section before "thermal efficiency" that talks a bit about how fuel is transferred into heat?  Perhaps a formula that shows it?  I'm a little confused...  Why is it that the same amount of fuel will raise the temperature of the exhaust gas the same amount even if the tempurature is lower?  It feels like if I was starting at -50c and going to 50c that would take a lot more energy than 25c to 125c.  Or am I thinking wrong?  Either way, adding a section about that would be fantastic, then I maymark your answer correct...

@JayCarr: Thanks for your encouraging comments, I am working on it ...

I like the addition, but I'm still a little confused on why the same amount of energy is needed from the fuel to heat the air, no matter what temperature you start at.

@JayCarr: The definition of the energy content of fuels is given as the capacity to heat one pound of water by one degree Fahrenheit. The definition of one Calorie is similar, but in metric units. Since the heat capacity of both water and air are almost constant at moderate temperatures, the starting temperature makes little difference to the absolute temperature increase when a given amount of energy is added.

Huh, didn't know that.  If you put it in the answer I'll mark this one as correct :)  (Sorry to be so finicky, but if I'm going to remove a well established answer, I want to make sure the new answer is as thorough as possible.)

@jCisco: This answer is about engine efficiency. The topic of optimum cruise altitude is discussed here.

You know, good sir, you should come join us in chat one of these days.  There are severally of us there who would love to pick your brain a bit ;). http://chat.stackexchange.com/rooms/12036/the-hangar

@JayCarr: I suck at chatting. I need time to research, type and refine a response.

Ah, we're all of varying levels of chatting ability (we have several non-native English speakers, for example.)  But I think everyone would be happy to see you there. It is up to you of course :).

What matters for a jet engine is the pressure and temperature differentials between the exhaust gas and the ambient atmosphere.  It is the expansion and high kinetic energy of the exhaust gas as it exits the engine that provides the thrust (and noise) of a jet (note this does not take into account the bypass portion of a turbofan).
The ambient pressure is atmospheric pressure, which for example at the surface is roughly 1000 hPa and at cruise might be 200 hPa or roughly a fifth of the pressure at the surface.  The temperature at that altitude is also typically around -50 C.
The exhaust gas pressure and temperature is controlled by a few things:

The compression by the N2 compressor stages -- Increases temperature and pressure
The hot section -- Greatly increases temperature and pressure
The N1/N2 turbine stages -- slight decrease in temp/pressure (work done on moving the turbines).

As the outside pressure is dropping as we climb, to maintain the same pressure differential in the engine, we need less temperature and pressure in the engine, and one way to do this is to reduce airflow into the engine and fuel added to that air.  The atmosphere takes care of reducing airflow (there is just less of it up at cruise, though this also depends on airspeed) and the FADEC takes care of adjusting the fuel flow.  The net result is less fuel needed to produce the same pressure differential when the air outside has a lower pressure, e.g. cruise flight.
EDIT:
Some of the other answers/comments make reference to mass flow through the jet, and particularly the mass flow through the exhaust nozzle.  I agree with that, but I didn't mention it directly because that mass flow is setup by the pressure gradient within the engine.  I should also clarify that pressure at the nozzle will be at or very close to ambient atmospheric pressure and it is the pressure gradient between that ambient pressure and that in the hot section that establishes the mass flow rate out of the engine.
Lastly, to address the bypass ratio comment, see Lnafziger's comment.  The turbofan engines on the EMB-145 are similar in that the bypass provides more thrust at sea level than cruise.  This perhaps relates to increased fuel efficiency at cruise in that the N1 fan is doing less work and thus the N1 turbine is extracting less energy from the engine.

the pressure differential between the exhaust gas an the ambient atmosphere should be a low a possible. It is not the gas exiting the engine that provides the thrust. It are the compressor and the combustion chamber that produce most of the thrust.

How then do the compressor and combustion sections produce the thrust if not through the modification of the air passing through the jet engine?  The compressor and combustion work on the air, and the air in turn works on the turbines and the atmosphere.

Does this mean that this answer, if modified, is correct?  ..some sort of reference might be wise at this point too.

http://en.m.wikipedia.org/wiki/Bypass_ratio

They work better at high altitude firstly because the air is cooler. Cool air expands more when heated than warm air. It is the expansion of the air that drives combustion engines.
The second reason is the low density of the air. Low density causes low drag and therefore the aircraft flies much faster at high altitude than on low altitude when it is given the same thrust. At this high speed, the mass flow through the engine is comparable to the mass flow at low speed in high density air (low altitude). The amount of energy needed is to heat the air to exhaust temperature is comparable between high and low altitudes. But since the aircraft at high altitude flies much faster, the amount of power generated is higher $(Power=Thrust\times{Speed})$ at altitude.
The difference with propeller aircraft is that at high speeds the propeller loses efficiency, and therefore the available power decreases with altitude.

You could also add the concept of ram air compressing to make this answer a bit more complete.

For a non-math approach:
Let's think how a jet engine works and compare low altitude with high altitude flights. The engine takes air from the intake situated at the front. As you are climbing, the air becomes less dense (there is less air mass in a volume) so you need to go a little faster just so that the mass of the air coming in through the intake is the same in a given second. You will actually get the same mass flow of air at high altitudes as you will get at low altitudes, but you are actually travelling faster.
Then you compress that air, remembering that as you are now travelling faster higher up, the ram effect will help you out and compress some of that air for you, just by 'ramming' your engines into it at high speed. As you compress it you pass it to the combustion chamber where it burns. This combustion stage is the same for both high and low altitudes, although the fact that at higher altitudes air is colder actually helps a little bit, as we can burn more fuel without reaching dangerous temperatures, so that's nice.
After burning it up, the air is passed through a turbine then expelled out the back. Now here it gets a bit complicated: You see, it is more efficient to accelerate a lot of air (mass) a little bit(small dv), than to accelerate a little bit of air(small mass) to a very fast speed(dv). This means in turn that the faster the airplane moves, the better the jet propulsive efficiency gets. So as you climb, you go faster and the flow gets better efficiency, plus the lower air pressure behind you means there is less of a force pushing against your outflow.
So what do we have in low vs high flight:
Same amount of air intake, same amount of combustion, same amount of fuel used, better jet propulsion at higher altitudes and better speed at higher altitudes. You just get more bangs for your bucks at higher altitude.
For a mathematical approach:
Jet Engine (Gas Turbine) Efficiency

I assume, though, that every engine must have an altitude of maximum efficiency then?  At some point the air is going to just be to thin to allow for maximized efficiency on the input, right?

no, not really: at some point the air becomes so thin you need to go very, very fast to keep the mass of air going in. And as you approach transonic speeds you get into trouble with a conventional engine. If you plan on a ramjet/scramjet engine you could go higher/faster, but you fuselage is now going to melt. Bottom line is: the top ceiling is never an engine-performance issue, it's always some other limitation

This is because the air is cooler and less dense which means that there is less fuel to air mixture at higher altitudes giving it a better fuel efficiency

The engine of an airliner is designed to be as efficient as possible over a journey containing a take-off, a climb, and most of its time at 35000 to 40000 feet where air pressure is about 1/4 to 1/5 of ground level.  The engine has a few extra compression stages to work efficiently at normal cruise at the expense of overheating if flown for long at full power near ground level due to too much compression at the intake.  Look up water injection for an interesting way to get take-off boost in a medium altitude engine in the 707.

The higher the altitude the thinner the atmoshere means less air resistance or  drag on the airplane so it needs less engine thrust to push it. Thats lucky because the engine loses thrust with altitude at almost the same rate because as less air is available to the engine, the fuel system must reduce fuel to maintain the correct air/fuel ratio to support combustion and keep the engine alight. Its a win win situation.

As you know as altitude increasing pressure and temp both are reducing up to stratosphere after that temp remain constant pressure drop continues so density of air reducing so its create less drag aircraft travel at high speed this pressure loss is overcome by ram pressure rise at the inlet of engine and aircraft required less power to move faster at 36,000 ft to 40,000 ft above more power full engine required to run faster so the blade
tip does not get stall.

Welcome to Stack Exchange! Please consider to rework your post; at its current state it is marred by spelling errors which make it hard to understand.

I fixed some of your worst spelling errors, but I cannot figure out where your sentences are. Sorry, but -1.

I think most are simply thinking too much.  The easiest and likely the most complete answer is resistance (or friction).  High altitude air is less dense, making it easier to pass through.  Oxygen content in high altitude is exactly the same as sea level.  While the air up there is the same air we breath, there is less of that air in the same volume container.
Space vehicles do not use jet engines.  To turn or do any movement actually they have "jets" in various locations around the shuttle.  "Jets" in this case are not jet engines, they are simply small nozzles that pressurized gases gets released through.  Keep in mind that with zero air, there is no resistance to movement, remember newton's laws of motion: every action has an equal an opposite reaction.

This answer would greatly benefit from a re-wording. Please consider editing your answer and explain in more detail the various arguments you bring forward. Consider also that formatting helps making an answer more readable

I'm not even close to an expert. But here it goes.
Air like water is thick. Submarines are slower than boats. Jets are faster than boats. Automobiles are faster than boats.
Space has no friction because there is no matter. But I think jets still work in space. Of course they need oxygen. Just like Superman doesn't need ground friction to run fast, while some other super heroes do. Which is why I find it unrealistic how a super heroes that need ground friction are able to run so fast and make sharp turns without doing serious damage to floors.
So I'm guessing that being that the air is less thick up higher, that the easier it is to travel through. The jets don't depend on friction like propellers do. Superman doesn't need friction like Flash Gordon or Wonder Woman. So in space, Wonder Woman would be helpless because her propellers don't work while Superman's jets will work just fine.
Of course jet's need oxygen. So I'm not sure how all that works.
And something I didn't think of was what was mentioned in another post. Sound needs air. So yeah. Sound can increase friction.

I don't know if I should vote you down for being ridiculous or vote you up for being creative.  I guess they cancel each other out, so I'll just leave it alone ;).

This post proves that there is room for art anywhere.

authors: Андрей Тарановский


text:
What was the first aircraft that could fly inverted?
I'm taking a guess and assuming the Wright Flyer was incapable of flying upside down.  I'm just guessing it probably lacked the power... ;)
So what was the first plane that could fly inverted for a sustained period of time (10sec+) with full power?  I'm assuming you would need an engine that was okay being upside down*, and had the power to get you high enough to make the maneuver safe.  Then you'd need an airfoil that worked inverted (some of the high early cambered wings may have had problems, for example) and controls that functioned in that environment as well.  Probably a seat belt would be useful too.
Anybody know what plane was the first one to have all of these, and then went ahead and flew upside down?
Bonus points: I also wouldn't mind knowing what plane was the first to simply fly inverted, even if for a brief moment...
*As Jan Hudec pointed out, the engine itself doesn't really care, but the fuel system might.  So really we should be asking when the first fuel system that could go upside down was produced...

The engine itself does not care whether it is upside down. The carburettor and the fuel system do. So whether the engine is radial does not matter (many early engines were actually rotary).

@JanHudec Ah, good to know.  I'll amend the question in a moment here.

The other factor is whether the wings are capable of flying inverted. Highly cambered wings (that were common on the early planes) may not be able to.

@JanHudec Added that thought in as well.

@JanHudec In addition to the fuel system the lubrication system often cares about the aircraft's orientation (at least on opposed engines like the ones many of us fly behind) As an example most of the Lycoming IO-360 variants are not designed for aerobatics/inverted flight - for that you would need an engine from the AEIO-360 series.

well, you COULD fly inverted in the wright flyer...  from cruising altitude until you hit the ground!

The answer is probably indeed A. Pegoud with a Bleriot monoplane in the fall of 1913, as answered by RedGrittyBrick. After that, a real craze started in Europe with stunt pilots flying briefly inverted at air shows.

The first purpose-designed airplane for inverted flight was probably a Grade monoplane which flew first in the spring of 1914. I have found this picture

on a Russian web page which lists lots of photos from early issues of Flight magazine. You have to scroll down 80% to find it. It gives the name as "Tweer monoplane", but it was actually built by the Grade aircraft factory right after the inverted flying craze was started by Pegoud. Gustav Tweer was an aerobatics pilot (one of many at this time), who was famous for diving his airplane almost into the ground at flight shows.

This airplane was designed for inverted landings, and with the uncambered wing I would expect that also the fuel system was adapted for inverted flight, however, there is no mention of that anywhere I have searched.

What a delightfully ridiculous flying contraption! I don't know if I'm more amused by the idea of landing it inverted or the scimitar propeller on what Wikipedia claims would be a 16HP engine.

@voretaq: These were the days when aircraft were not designed for war, but for air shows. The Wikipedia article concerns the first Grade monoplane from 1909, and I would expect that the inverted-V four cylinder engine in the picture above was a different engine with more power.

@PeterKämpf Wow, that's amazing.  This certainly looks like it was designed to fly upside down, so long as nothing came before it, it must be first, right?  I just wish we had some details on it's design so we could know what they did to make it good for flying upside down (aside from the crazy landing gear configuration.)

I traced the picture to Flight 1914 page 399, but there does not seem to be anything more except the image caption transcribed on the Russian site too.

umm, is that propeller ... backwards? and reversed .... and....wtf? Gonna have to do some research now, for sure.

Further search has to be done by somebody who understands German, since most references about Gustav Tweer seem to be German only.

@CGCampbell: It is neither reversed nor backwards. It is normal right-hand (clockwise rotating) propeller and just like modern scimitar propellers the leading edge is curved.

@PeterKämpf "Wars are just airshows with bigger fireworks displays!"

Deliberate momentarily inverted flight
The first reported deliberately inverted flight was probably in a Blériot model XI.

Inverted flight was also being considered. Several pilots had inadvertently found themselves upside down as a result of wind gusts, but no one had yet attempted it intentionally. Adolphe Pegoud decided he would be the one to try.
Pegoud was one of the first true aerobatic pilots. An accomplished test pilot with the Bleriot team, he was also the first flyer to jump from a plane with a parachute. Both Pegoud and the aircraft landed, unharmed. Having decided to try upside-down flying, Pegoud first practiced in a hanger [sic] in a plane hung upside down from the ceiling. He correctly assumed the controls would have to be operated in a reverse manner and he wanted to get the feel of it before actually flying.
It must have worked. Shortly thereafter, Pegoud made the first public demonstration of inverted flight in September of 1913. During this flight, he was the first to perform a half roll to an inverted position--another aerobatic maneuver was born. The flight did point out one potential problem: While Pegoud was prepared to fly inverted, the aircraft was not. It proceeded to drench the flyer with fuel!

From The history of aerobatics
Deliberate sustained inverted flight
This would have to have been an aerobatic aircraft specially modified for inverted flight

flop tubes in the fuel tank?
fuel-injection not carburettor?
strengthened and/or braced to allow wings to support fuselage in inverted position?

1925
June 1925, No 32 Squadron did an air display demonstrating Flight-converging bombing at the R.A.F Display, Hendon. Scott was selected to do individual aerobatics in a brand-new Snipe which he was allowed to paint red, this pleased Scott greatly as it meant that he was also allowed to practice his aerobatics at a low altitude, rather than above 2000 feet which was R.A.F regulations at that time. In Scott's book he tells of how for his solo display he was allotted exactly seven minutes during the luncheon break, to complete his show but after just two minutes, a flying wire broke in the near edge of the port side, anxious not to cut his allotted seven minutes down too much he began to fly the aircraft upside down in an effort to reduce the strain on the flying wires, he continued flying in an inverted position for some time until he noticed a worrying quiver in the top plane and promptly landed slightly short of his seven minutes.
From Wikipedia
1933 record
In 1933, NC872H, a Boeing 100, was flown inverted for 4 hours 5 minutes and 22 seconds.

After its career with Pratt & Whitney, the Museum of Flight's aircraft was sold to Milo Burcham on September 27, 1933. He modified it for skywriting and air-show work, painted it blood red, added fuel and oil systems for inverted flight, and flew it upside down from Los Angeles to San Diego.

From Seattle Museum of Flight

lol, yeah, definitely not a plane that was designed with it in mind.  But hey, you got the bonus question it seems :).  Well, the first one anyway, that second one was ready for it, but it sounds like that plane was far from the first.

Lots of really good information @RedGrittyBrick, but I think Peter answered the title question so I marked his as right.  Apologies, I really enjoyed your answer!

During WW2 the RAF used carburettors. They operated on a swivel system which which would allow them to rotate towards the ground and operate normally


text:
Is it legal to drop/shoot paintballs from an airplane?
My brother and I are big into paint balling and aviation, and we've often wondered if it would be legal/possible to use an aircraft in a large scale paintball match?  Mainly to drop paintballs, or perhaps even bags of paint, onto participants to simulate air support.
A few questions then, to cover this fully:

Is it even legal to begin with?
Assuming it is legal, what is the
recommended way to do this?  Plane size? Configuration considerations?
Lastly, is it legal to shoot back at the
plane?  Or is that just wildly unsafe?

I know, it's kind of a broader question, but I was hoping to get the entire answer in one place for ease of access to the answer.

Idea: Get a 3m radio airplane model, and make it a working variant: camera + bomb dropper. Cameras are a standard thing, bomb dropper can be a simple relay-controlled electromagnet, controlled e.g. by GPIO pins on a Raspberry Pi or Arduino.

Wouldn't it be a lot cooler / simpler / more logical / more legal to simply drop a "paratrooper" who happens to drop something out of his pocket on the way down assuming he feels its safe etc?

@VinceMulhollon mmm, probably not simply because we play in wooded areas, so I don't know that we could guarantee a safe landing...  Same problem as with a low flying plan actually.  I think the quadcopter idea is my favorite at this point.

Sounds a little dangerous and a lotta fun!

Greg covered this very well in his answer.
The "miss by a surprising distance" bit is especially important here in the context of FAR 91.15 -- you would have to have a VERY large-scale paintball match for this to be viable, and some specific rules of engagement for the aircraft (always firing "into" the arena area as you don't want to miss and send a paintball into someone's car or kitchen window as that would get some distinctly unfriendly attention from the FAA, and probably the local constabulary).
Operationally, if we're doing this with an airplane high-wing aircraft would obviously be the order of the day (Cessna 152), with a pilot (whose sole job is to fly the airplane) and a gunner/bombardier (who is responsible for hitting targets on the ground). You really don't want the pilot distracted by the game.
Your weapons guy is going to have to be able to account for the aircraft's speed and altitude in targeting (which is pretty hard with an adjusted scope/bomb sight - doing it without one is going to be tough).
As for shooting back at the plane, if we're talking about a "regular" airplane (something manned, like a Cessna 152) I would advise against allowing players to target the plane. At safe operating altitudes they're unlikely to hit the plane, but if they do aircraft skin is very thin & dent repair is very expensive: If someone is lucky enough to score a hit it could be a costly "death".

Now, as a paintball player myself I will tell you that I find the idea of death from above when I can't shoot back at the aircraft to be rather unsporting!
What I would suggest instead is to deploy quadcopter drones. These can be had pretty cheaply and with some basic electronics skills they can be modified to carry/drop paint grenades (think Squadbusters) on your enemy.
If you want to get really fancy add a GoPro or some other remote camera to help with targeting, or add a paintball gun to the thing like these folks did.
The FAA's rules for "unmanned aerial systems" are still being written, but if you keep to the general rules for model aircraft (below 400 feet, and not charging anyone money for the drone's services) I think you'll be OK (when the final rules come out you'll have to review your drone for compliance).
As a bonus if the drone isn't too large and you have an indoor arena with liberally-minded management you may even be able to use your drone during indoor games.
(Kill rules for the drone and its operator are left as an exercise for the players.)

Yeah, quadcopters might be the better answer here easier to keep local...  Hadn't really thought of that.

For the FAA:

§ 91.15 Dropping objects.
No pilot in command of a civil aircraft may allow any object to be dropped from that aircraft in flight that creates a hazard to persons or property. However, this section does not prohibit the dropping of any object if reasonable precautions are taken to avoid injury or damage to persons or property.

It is legal to drop objects from an aircraft as long as no hazard is created. It's open to interpretation, but I would suggest that deliberately dropping bags of paint onto persons on the ground would be a hazard.
Also, having dropped objects (pumpkins) from an aircraft attempting to hit a target, I can tell you that it's really hard. You will most likely miss, probably by a surprising distance. If you do try this, make sure you have a lot of room. Also keep in mind your required altitude minimums, as you must be able to land safely if your engine fails at any time.

I would say that if everybody participating in the paintball match were aware of it and expecting it that it is not creating a hazard, especially with "reasonable bombs".  Otherwise, I agree with you!

The AMA guidelines for RC planes has the same wording, so a drone strike would (or at least should) have the same restrictions.

Also, terminal velocity of a paintball is definitely less than what the gun fires them at, so it may be even less of a hazard.

Dropping something like a paint grenade would be pretty safe - the bigger worry in my mind (from "real plane" altitudes) would be the grenade rupturing on impact rather than spraying like it's supposed to.

Is it even legal to begin with?
I'm going to say no. Yes, 91.15 does allow the dropping of objects:

No pilot in command of a civil aircraft may allow any object to be dropped from that aircraft in flight that creates a hazard to persons or property. However, this section does not prohibit the dropping of any object if reasonable precautions are taken to avoid injury or damage to persons or property.

But 91.13(a) says.

(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.

There is also 91.119(a)-(c)

§ 91.119 Minimum safe altitudes: General.
  Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

For an aircraft to be at all involved in a paintball match, it will need to be doing low altitude maneuvers. This will violate one of the above laws. I'd say the most assured is 91.119(a). An engine failure in that situation would be very dangerous to the people on the ground.
I absolutely believe, based on my experience with the FAA investigators, they would not consider this legal.
Assuming it is legal, what is the recommended way to do this? Plane size? Configuration considerations?
My very strong recommendation is that you never, ever do this.
Lastly, is it legal to shoot back at the plane? Or is that just wildly unsafe?
18 U.S.C. § 32 : US Code - Section 32: Destruction of aircraft or aircraft facilities says you can get 20 years for damaging an airplane. I'd also say it's wildly unsafe.
I say this all as a pilot who has dropped his share of stuff from airplanes. Rubber chickens onto a toy house at an airport as part of a competition, toilet paper to tear up with my prop, pumpkins in the marsh and paper bags filled with flour. There is a time and a place for it, and a paintball match isn't it.

Excellent points @Jungroth.  After reading this and Vortaq7's answer, I think I'll be looking into using quadcopters instead.  Smaller, nobody on board, you can theoretically shot back (so long as you're willing to risk the quadcopter...)

91.119 a & b would seem pretty easy to comply with if the paintball field has a clear area sufficient to land the aircraft but that's starting to impose a lot of restrictions on the paintball match. It's also pretty likely the FAA might consider it "careless & reckless" under the 91.13 catch-all (about the only way to avoid that is to get the local FSDO to sanction the event, which is pretty unlikely).

@voretaq7 FSDO?  Whats that?

@JayCarr Sure, make me write the [tag:fsdo] tag wiki :-) Flight Standards District Offices (FSDOs) are basically the FAA "branch offices" - they handle things like coordinating airshow/exhibition permits, waivers, and authorizations (which is what this would probably be most similar to).


text:
Why are Inline Engines more commonly used than Radial Engines?
Back in the early days of aviation radial engines were very popular, it seemed like all of the big planes of the 20s, 30s and early 40s used them.  But some time during WWII (or so it seems to me) aircraft manufacturers switched from radial engines over to inline engines.
Why was there a switch?  Why do modern manufacturers rarely, if ever, use radial engines?
Example of a Radial Engine for those who are unfamiliar:

there is only so many cylinders you can put on a radial before the prop becomes a useless prop (pun very much intended)

The one above has 7 cylinders, a Cessna 172 has less than that, doesn't it?  Perhaps on bigger planes then, but on small ones...

This isn't really a full answer, but one limitation of radial engines is that they have to have an odd number of cylinders (without heroic design measures). It's due to the 4-stroke timing pattern.

@dvnrrs - odd per row. There are 18 (2x9), 14 (2x7), 28 (4x7) arrangements and more! Many have even total numbers of cylinders.

@egid Point taken, I did mean by row.

Looking at a photo like that, how can you tell at a glance that it's a radial engine and not a rotary engine? I'm still trying to wrap my mind around how a rotary worked -- were they 2 stroke and the ignition system rode along with the crankcase and cylinders?

@PhilPerry In a nutshell, radials have pistons (like that one does) and rotaries have a giant cylinder.  Further info: https://www.youtube.com/watch?v=6BCgl2uumlI

No, that's not the "rotary" I'm talking about. Rotaries had the crankcase and cylinders spinning, with the propeller attached to the crankcase and one end of the crankshaft anchored to the rest of the plane. I believe that some small planes have been powered by the Wankel Rotary Engine, which sounds like what you're talking about.

@PhilPerry So basically the whole engine would have spun then?  Well, I know the engine shown above doesn't do that...  I was unaware that there was an engine that did (it sounds pretty inefficient to move the hundreds of pounds of the engine rather than just the prop shaft...).  But I'd be very curious to see the plane that does that, that's for sure.

I believe that rotaries were very popular around WWI vintage. Yes, spinning such a large mass could really make the handling of an aircraft very tricky (counter torque and gyroscopic effects).

@PhilPerry I did find more information on what you were talking about, btw: http://en.wikipedia.org/wiki/Rotary_engine

Your premise is not true. Aircraft built for speed (like Supermarine S6) had inline engines long before WWII and on the other hand all US Navy fighters had radial engines throughout the war and so did all US Army bombers and transports. And that remained the case until introduction of jet and turboprop engines.

The big advantage of radials at the beginning was their large frontal area, which meant they could be air cooled. An inline air cooled engine can run pretty hot on the rear cylinder. The bigger the engine, the more of an issue cooling becomes. There are some huge air cooled radials, in sizes that would be unthinkable for an air cooled inline engine.
As technology advanced, more complex water cooled engines became more popular. However air cooling was still a big advantage in military planes, due to the absence of a delicate cooling system that could be damaged, enabling radials to persist in this arena. Of course these days no-one would dream of putting a piston engine in a fighter.
Nowadays liquid cooling is almost universal for piston engines in general applications, with small aircraft being one of the few areas where air cooling has managed to hold out to some extent (see below.) In the absence of a military application that prefers air cooling, the large frontal area has become the radial engine's downfall, due to the large aerodynamic drag it produces. The other problem is that the valve gear is quite complicated, which means overhead camshafts and multiple inlet/exhaust valves per cylinder are impractical. This makes efficiency vs power to weight ratio a big issue as follows:
For an engine to have good power to weight ratio it must run at high RPM. To run at high RPM it must breathe effectively. To do this with just two valves they have to open wide. To stop them colliding with the piston, the compression ratio must be low, which limits efficiency. Hence most modern engines have more than two valves per cylinder to get around this problem.
Further reading
Pratt & Whitney R-2800: 46L displacement, over 2000hp from an air cooled engine, by no means the biggest!
Excellent animation of the valve gear, which also tends to illustrate the difficulty of putting more than 2 valves on a cylinder. Also, lots of history (obviously.)
In World War II both Double Wasps (46L air cooled radial) and Merlins (27L water cooled V, licensed from Rolls Royce in Britain and built locally by Packard) were used by the American military. I once read a very good authentic World War II discussion about the tactics of when to use air cooled radial fighters and when to use water cooled ones. Unfortunately I can't find it. Here's one of the many discussions on the subject. Note that the Merlin had overhead camshafts and 4 valves per cylinder, like modern engines, whereas the Double Wasp obviously did not.
Modern air cooled engines
I am more of an engine guy than a plane guy, but I've just been looking at modern air cooled aviation engines. They are only available in much smaller sizes than typical WWII fighter engines. Typically they have 4-6 cylinders, too few for a radial, but enough to cause cooling problems on the rear cylinder in an inline.
The compromise is a flat (boxer) engine with 2 or 3 cylinders on each side. This also overcomes two other problems with radials: upper cylinders obstructing pilot view and oil collecting in the lower cylinder when stopped. You also have at least one of these problems with an inline, depending if you mount it upright or inverted.
Small aero engines can be either water cooled or air cooled. The designer has to ask: given the issue of reliability in aviation, does the weekend pilot really want an engine as complicated as the one in his car, with water cooling, cambelts and engine management, all of which could go wrong?
The air cooled engines I've seen (Lycoming) do only have two valves per cylinder, all driven from a common camshaft. (A simpler choice than the cam ring on a radial) I haven't seen an air cooled flat engine with overhead cams and multiple valves per cylinder, but it would be easier achieve than doing the same thing on a radial.

British and German WWII designs had few, if any, air-cooled engines. Most were inline or vee, some inverted. On the other hand, Americans were very fond of air-cooled radials, especially on bombers and early fighters. Most of the US fighters eventually went to water-cooled inline and vee designs.

@PhilPerry: British used radial engines (mostly Bristol-made) on many bombers. Germans did not, indeed, seem to use radials at all. Americans (and Japanese) kept using radials on carrier-borne fighters presumably due to their ruggedness.

@Jan: The Focke-Wulf Fw 190, one of the most prominent German WWII fighters started out with a radial engine and got it replaced only in the late-war D-models by an inverted V engine.

@Scrontch: You are right. I checked it and saw that it used in-line engine too, but missed the fact that it was only a late modification. It was still an exception though as even German bombers were mostly powered by water-cooled in-line engines.

Some of the fastest, most powerful piston-engine fighters ever built were radial-engined: the Hawker Sea Fury and the Grumman Bearcat.

There are stories of radial engines continuing to operate after losing a cylinder - as in blown off. Comparable damage to a liquid-cooled engine would be fatal as the entire engine would lose cooling. That would have been a factor in warplane design when assessing ruggedness.

It's hard to find a general pattern in radial vs inline engines in WW2.  Each country had engine manufacturers that produced the engines they were good at.  An important consideration was how the engine performed at altitude, which required complex superchargers or turbochargers.  In the US, the radial was far more developed.  The only US inline engine to see much service was the Allison V1710, which lacked a quality supercharger.  Turbocharged V1710s powered the P-38, making it an effective fighter at altitude.  The Pratt and Whitney R2800 radial was used in the Hellcat, Corsair, and P-47.  The Hellcat and Corsair were supercharged and performance fell off above 20,000 feet, not a problem for a naval fighter.  The P-47 was turbocharged.  The great size of the Thunderbolt was due to turbocharger ducting leading from the engine to the intercooler and back to the engine.  The turbocharger allowed the P-47 to be effective at high altitude.  The P-51 switched from the Allison V1710 to the Packard V1650, a license built Rolls Royce Merlin.  The V1650 had a better supercharger for altitude performance.
In the UK, Rolls Royce produced the superb Merlin.  It was so good that the Seafire and Barracuda carrier aircraft used it.  As the war progressed, RR produced the Griffon, Napier the inline Sabre, and Bristol the Hercules and Centaurus radials.  All of these engines were capable of driving a high speed fighter.
Germany switched the FW-190 from radial to inline in the D model in search of altitude performance as the BMW radial lacked a good high altitude supercharger.  The original FW-190 owed its existence to not using the valuable inline engines needed for the Bf-109.
Japan and Italy never developed effective inlines.  The Ki-61 fighter was mediocre with a German designed inline and became the effective Ki-100 with a radial.  They also switched the D4Y dive bomber from inline to radial.
While the inline was more streamlined the radial could use the air cooling to produce thrust.  The air comes in cold and leaves hot (like a jet).  The radial saved weight by eliminating the liquid cooling system but the inline could be run more intensively because the engine cooling was more effective, resulting in smaller engines for the same power output.  The radial was less vulnerable and eliminated the logistics of liquid coolant.  By the way, about 50% of the cooling in a radial is from the oil, so even a radial is partially liquid cooled.
After the war, every airliner before jets used radials with one exception (I can't remember which) powered by the RR Merlin.  Modern manufacturers don't need the massive power in the smallest weight and volume possible so they choose simpler designs.
It seems that the aircraft designers used what they had and only switched from radial to inline or vice versa when forced to.  The companies that started the war making inlines continued as did the radial manufacturers while aircraft designers used what they could access.

When I was at the "Kamikaze" museum near Kagoshima, Japan, a few years ago, they showed the Japanese inline, a copy (probably licensed built) version of the German DB601, as used in early Bf109.  As noted, very few Japanese aircraft used this engine, and the museum information card stated that one reason was that the inline engine required much more precision in the machining of parts compared to the radial engines normally used in Japanese aircraft, and Japanese industry was not equal to the task, during the war, of large scale production of a technically sophisticated engine like the DB601.

...every airliner before jets used radials with one exception (I can't remember which) powered by the RR Merlin. Were you thinking of the Avro Lancastrian?

Radials tend to have a great power to weight ratio, but they are still heavy and complex.
Another big issue is their surface area. Compare a 300HP radial to a 300HP  opposed 6-cylinder and you'll see that the radial probably has 9 cylinders and a big round crank case that must sit upright, facing into the air stream. That causes a lot of drag. The opposed engine, however, can be placed inside a small aerodynamic cowling with openings only for cooling air (or none, if you water cool it).
I think what it all comes down to, however, is that radials are great at producing loads of horsepower and torque. You can stack radials up to something like 50 aircraft-engine-sized cylinders before the friction causes diminishing returns. But what aircraft on the market today needs 5000hp and a two-ton engine? Radials that produce lower HP can be replaced by significantly smaller opposed engines that work just as well or better, and radials that produce large amounts of HP can be replaced with turbine engines that vastly outperform them on the higher end.
The only real use for a new radial engine today is to hear the sweet music they produce.

I remember when "FiFi", a B-29, rolled into my town...  That was a beautiful sound :).

Radial engines are better from the durability/mechanics point of view: having radially distributed pistons distributes the loads better and the shaft will be stressed equally from all directions.
For resonance reasons you will (almost) always have an odd number of pistons.
Times have changed and manifacturing techniques have improved*. The benefits of the radial design justify no more the additional complexity and design effort required**, but even then some are still produced today. Also, in some cases engines are simply car engines re-adapted for an aircraft, cutting sensibly on development costs.
As Ratchet mentions, you have also the limited space for the pistons radially arranged, something that in the linear arrangement is limited only by the length available (and you could always switch to an H arrangement to double the number of pistons). Even if some multi-row radials exist.
On wiki they also raise some interesting points concerning mostly the cooling system, but also concerning placement of the engine and pilot field of view.

*: mostly metallurgy (better materials) and construction techniques (less imperfections in the components). Also, a better understanding of the science behind combustion has led to more efficient designs of the combustion chambers that, together with the fuel, has increased the overall efficiency of the engines.
**: on a radial engine you have the pistons all around (duh) and thus you need to have a distribution line that reaches all sides of the engine (one side only for a standard inline) and an injection system , especially for the bottom pistons, that does not suffer from being "upside-down" for all the time. Something that the inline approach does not (normally) need to take care of.

I wonder if you can be a bit more specific by what "manufacturer techniques have improved" and "complexity and effort of design"?  I'm hesitant to mark this as the right answer without that info.

@JayCarr added some information. As for the specific improvements on metallurgy/fabrication techniques, I feel that they are quite off-topic here.

Federico, re: your ** footnote.  There are many in-line engines that are inverted.  Wikipedia lists 39 of them.  These are all WW-II vintage powerplants and obviously are not in general use today.  So the "inline approach" does "need to take care of."

bottom cylinders can get oil that leaks above the piston after setting idle. If this is not managed, damage will result.  They walked the prop through by hand in old wwII planes to clear this oil.

Pulling radials through by hand doesn't clear the oil, it just provides a way to feel the oil ("hydraulic lock") without applying enough torque to seriously harm the engine: The starters could push the piston against the oil hard enough to bend connecting rods, and if the engine fired damage was virtually certain, but a person could stop when they felt too much resistance. The oil is typically drained off by removing one of the bottom spark plugs. You'll see planes with radial engines today that have with a bottom plug out and a drain hose going to a bucket for this same reason.


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In a piston aircraft, how much noise is generated by the engine, and how much by the propeller?
I've been reading quite a few articles lately about how noise complaints from GA airports across the country.  One of the common suggestions to fix the issue is to simply use an electric engine instead of a gas powered one.
Leaving aside other practical concerns (batteries and weight for example), would this actually significantly reduce the amount of noise a GA aircraft makes?  I'm told a lot of the noise actually comes from the propellor itself, not the engine.
So, in essence:  How much of the noise from a GA aircraft is made by the engine, and how much is the propeller?  And does the percentage change depending on the phase of flight?

Note: I am not asking for subjective answers here.  My hope is that someone actually has a decibal break down of how much noise comes from the engine, and how much from the propellor.  Yes, I know it will vary by craft.  An average would be fine, or perhaps numbers from a popular GA craft (like a Cessna 172 or Piper Cub, for example.)

Using electric engine and ducted fan is more like to eliminate the issue than to solve it. For what we usually define as a "solution" it just need to be good enough rather than perfect. But even if you make all airplane silent, if someone choose to ride a loud motorcycle by the airport you will still be receiving complaints. Prop or fan, piston or electric, all depend how aggressive the noise reduction is required.

have a look here https://aviation.stackexchange.com/questions/32298/why-are-light-aircraft-engines-so-noisy

@ymb1 They do.  This question isn't really about that particular debate though, I'm just curious if using an electric engine would actually make an appreciable difference.

@chaos505 - I'm aware.  I was hoping someone might have an actual break down.  Perhaps I should be a bit more specific in how I ask the question.

I think that the sounds generated by the engine and the propeller actually end up at different frequencies most of the time. The engines produce a hum at a lower frequency like many of the other little-muffled piston engines on lawnmowers and snowblowers. The prop seems to generate a much higher frequency cracking-like sound. Based on this, I would probably guess that there would be two separate peaks if you went to analyze generated by a prop airplane.

I think the noise heard directly below a small aircraft such as a Cessna 172 is mostly due to the engine, because the exhaust is relatively short and not a lot of muffler and the exhaust points straight down usually.  Additionally, I've noticed aircraft with constant speed props make a lot more noise at high power than low power even when RPM is relatively constant.

@SMSvonderTann That's an interesting point.  If that's true, perhaps I can take my own audio equipment and figure it out on my own :).

One particular example I can recall is Extra 330. Modification with electric engine have been built (Extra 330LE) keeping the rest of the aircraft quite similar to the original petrol-engine version as far as I know. According to this article electric version was measured to produce 14.5 dB less noise compared to the combustion engine aircraft.
Maybe you can try too look for other combustion engine aircraft converted to electric without making other significant modifications (I am not aware of any other than this Extra 330LE though) as a source for data you are looking for.


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Why does this Garmin GMA340 have an Airway/Inner marker indicator?
On this Garmin GMA 340:

the outer marker is blue and is designated O
the middle mark is amber and is designated M
the inner marker is white and is designated A and placed on the far left.

I looked at the GMA 340 Manual and could not find an answer although it does say Airway/Inner Marker.
Why would it say Airway if this is what the A indeed stands for and why even have an inner marker because that Garmin can't do a Category II approach?

It was a typo yes, thank you.

The "A" is not an inner marker. The A is an airway marker. Airway markers were part of a now-obsolete system of "red" airways. They were placed along the enroute path and they could be picked up at cruise altitudes. The red airway system was mostly abandoned in the 60's and few "A" markers are still operational. Here's a map showing where they were located. They weren't at aerodromes.

Source
The airway markers transmit the same 3000 Hz audio frequency as inner markers so the airway marker can be used to pick up the inner marker. Since you say that unit is not certified for cat II it makes sense that they labeled it "A" instead of "I" although it can be used for inner markers.

red airways are not obsolete: http://aviation.stackexchange.com/questions/11737/what-is-a-red-airway-on-the-vfr-sectional

@rbp Ah so. I'll correct. Somebody should correct Wikipedia too. Aside from Alaska and Florida are they still used elsewhere? And are the beacons still operational?

You have mentioned that outer marker is designated as B which is not. It is 'O'. Now the question is if O is outer marker and M is middle marker then what is indicated by A? A serves two purpose, Inner/ Airway marker. So A does not indicate only Airway marker.
The airway marker was used to indicate reporting points along the center line of now obsolete "Red" airways which  Tom McW mentioned.
This was sometimes a "fan" marker, whose radiated pattern was elongated at right angles across the airway course so an aircraft slightly off course would still receive it. Please do not confuse about fan marker, The term fan marker refers to the older type of beacons used mostly for en-route navigation. Most of these are obsolete now a days. There are uses of marker beacon on the instrument approaches mainly.

Yep, I should have typed O. Thanks for the correction. Thank you for the added  info about airway/inner markers.


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What is the difference between an air carrier and a commercial operator?
I am currently studying for my commercial and I've been seeing both of these terms used interchangeably. Is there a difference?

From 14 CFR 1:

Air carrier means a person who undertakes directly by lease, or other arrangement, to engage in air transportation.

Commercial operator means a person who, for compensation or hire, engages in the carriage by aircraft in air commerce of persons or property, other than as an air carrier or foreign air carrier or under the authority of Part 375 of this title. Where it is doubtful that an operation is for “compensation or hire”, the test applied is whether the carriage by air is merely incidental to the person's other business or is, in itself, a major enterprise for profit.

To make this more clear FAA Order 8900.1, Volume 2, Chapter 2, section 2 says:

2-128    TYPES OF CERTIFICATES UNDER PART 119. There are two, basic types of air operator certificates (AOC) issued to U.S. applicants who will conduct operations in common carriage. See Table 2-4, Certification for a summary of certificate requirements.
A.    An Air Carrier Certificate. This certificate is issued to applicants that plan to conduct interstate, foreign, or overseas transportation, or to carry mail.
B.    An Operating Certificate. This certificate is issued to applicants that plan to conduct intrastate transportation.

From your perspective it isn't a too important distinction.  As a commercial pilot all that matters to you is that both air carriers and commercial operators need to have operating certificates.  As a commercial pilot you can be hired by someone who has an operating certificate but you cannot provide the same services on your own unless you yourself have an operating certificate (e.g. a charter operator can hire you, but you cannot operate a charter service).
As a commercial pilot you can be paid to act as PIC and can engage in the commercial operations that are excepted from operating certificates in 14 CFR 119.1(e).  You cannot act as either an air carrier or a commercial operator without complying with 14 CFR 119, which requires that you have an operating certificate.


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How do drones overcome latency?
The US Military's use of drones has become commonplace, and widely known at this point. Although these vehicles are operated from nearby bases during these attacks, it seems like the latency (time of transmission) would be problematic as far as flying goes. Are these vehicles typically flown 100% by autopilot, where there's very little real-time reaction?

What latency?  Radio waves can travel around the earth about 7.5 times per second.  Let's imagine your drone is 10,000 miles away - a long way - a radio signal can travel there and back about 9.5 times per second.

@Simon so, that's a 100ms lag. talk to any gamer and tell them that's not a huge latency. (of course you are not factoring the lag added by the various transmission equipement.)

@njzk2 I don't see the correlation between gaming and drone flying?  Is there one?

@Simon: Online gamers, especially shooters, have to deal with lag when controlling the game. Their controls are quite similar on a broad level to drone controlling, as there's essentially a feedback loop separated by connection lag from the actual system.

@Simon: For gamers, the latency is not the result of the distance but of the delay in the equipments along the packets path (routers, amplifiers). Companies like Akamai provide low latency routing (for a price). For a drone the latency would come from the distance of the satellite(s) relaying the commands.

100 ms lag is probably negligible in this case.  this isn't an FPS shooter.  Go fly a cessna and observe the latency between providing a control input and the airplane doing anything and you'll find that is greater than 100 ms.  As a pilot you learn to anticipate and that is how you overcome lag.

Additional latency will be incurred from image transmission. For example if the on-board camera produces 50 frames per second and any kind of compression is used, then the process of compressing an image (by comparing the full image with previous images) cannot really start before the image is available and similar on the decompression side, leading to at least one frame-time additional delay.

@Simon, Keep in mind that once you reach a distance where you have to bounce the signal off of a satellite, you're looking at 45,000 mile minimum to get the signal into orbit and back down.  Once you're out of line-of-sight distances, lag jumps to a point that makes takeoff and landing nearly impossible unless the drone just does it automatically.

@JPhi1618 and others.  The drone is controlled locally for take off and landing.  For remote flight, the latencies are insignificant.  This isn't Call of Duty.   When loitering or enroute, you make a control input, half a second later the drone responds.  So what?  That's far better than many older and heavy aircraft.

@HagenvonEitzen: MPEG type compression processes blocks of say 8x8 pixels, it doesn't need to have the full image to start.

There are multiple ways to fly and control Unmanned Aerial Vehicles (UAV), or drones, for e.g. the Predator.

Source: science.howstuffworks.com

Some UAVs (like the Global Hawk) can perform the entire mission (from takeoff to landing) autonomously, negating the need for a pilot.
Most of the UAVs, like Predator can perform (i.e. fly in) some simple missions (like reconnaissance) autonomously.
In case a geostationary satellite is used for communication with the UAV, the latency is around 300 ms (The electromagnetic waves have to travel ~70000 km). In case of other delays (like equipment etc.) this will not be greater than 500 ms or around half a second. This is not significant in case of slow moving systems like Predator (they are not dog-fighting, after all) and the 'pilot' can effectively control the aircraft.
The only situation where this is a problem is when the UAV is taking off/landing and in this case, the control is handed over to local LoS operators.

The Predator is reported to have a 'latency' of around two seconds, which causes problems during takeoff/landing. According to the telegraph,

But the two-second delay between a pilot moving a joystick in Nevada and an aircraft responding in Afghanistan is enough to cause a crash during take-off and landing. Crews in Afghanistan control 'launch and recovery’ through direct contact with antennae on the aircraft. Half an hour after take-off, control of the Reaper is handed to a crew in Nevada; half an hour before landing, it returns to the crews on the ground in Kandahar.

Note that the RAF also does things the same way. According to the report on formation of 'drone' squadron' at RAF Waddington,

Although the personnel will be permanently based at RAF Waddington, some will go to Afghanistan to control the take-off and landing of the drones locally.

If there has to be a local operator for takeoff and landing why doesn't that person flythe entire mission?

I think your latency figure is one-way, but unfortunately, because it takes time for the updated instrument signal to travel from the drone to the pilot, and time for the control signal to travel back to the drone, you have to measure it two-way, round-trip.

@TomMcW It's cheaper to have one guy in Afghanistan launch a couple of airplanes and a bunch of pilots in Nevada than ship the bunch of pilots to Afghanistan - after some distance, communication is going via satellite either way. Besides, the guy in Afghanistan has a risk of being hit by things that go boom, the guy in Nevada not so much.

@sanchises Certainly the point of having the pilots in Nevada takes a person out of the line of fire, but having to have someone there qualified to take off and land seems to negate that.

@TomMcW You're missing the point that the one guy on the front line can launch and land all the drone missions because he only has to do that one thing that only takes 30 minutes.  Once launched, the guys in Nevada can each control ONE drone for a multi-hour mission.

A two second lag is unbelievably bad! I play some really complex video games on Xbox live against people all over the world, occasionally even against soldiers IN Afghanistan. There can be twenty or 30 different players from different continents interacting in real time. You get some latency but as was noted in earlier comments, even 100ms of latency is very noticeable. 500ms would be enough to stop playing and wait till your network clears up. Maybe the military needs to use the Xbox servers!

@TomMcW: There is little to no relation between what a Predator drone does in real life, and what you do with your little first person shooter computer games in your bedroom.

@Lightness The first-person shooter probably requires several times the data and processing power of the drone. The big difference is the security necessary. (well, that and the fact that people aren't actually dying when you shoot a missile). Don't get me wrong, I'm not trying to say that since I can fly a drone on my Xbox that I can fly a real drone. The point is that it is quite possible to move very large amounts of data around the world in milliseconds. To communicate control inputs from Nevada to Afghanistan could not take 2 seconds. That's prob just what the Telegraph was told.

@TomMcW Get satellite internet and you'll find your games unplayable. If we had the luxury of laying millions of miles of wiring all over the potential battlefronts around the world so we could then erect antennae all over to maintain constant line-of-sight contact with the drones, the amount of latency would be comparable to using the internet to play games. But trust me, I've tried, and you just don't get good latency on satellite connections (although you could get really awesome bandwidth).

@tpg2114 I've read that satellite internet makes for long latency times. Not sure why that is, tho. All I know is that I've played battlefield with a bloke from Australia (I'm in Kansas, US) and there was latency, most noticeable on the voice chat chanel, but more in the 250-300 ms range. And i dont think even Microsoft can run a cable from the us to australia. Gotta be via satellite. Just can't imagine that dedicated military hardware and satellites would be 8 times slower.

@TomMcW It wasn't Microsoft, but it was a Bermuda based company that ran a cable from the US to Australia. Satellite communication makes for long latency times because geosynchronous orbit is pretty far away and the signal has to go out and back before it gets to you (plus all kinds of cables, hardware, and software in between). 8 times slower is not that hard to believe.

@tpg2114 Man, that's a lot of cable! Had no idea. I stand corrected. But i still think 2 seconds is pretty long. Was looking at a website for a satellite isp a few min ago (can't seem to find it in my history) that was explaining that customers should expect 500-750 ms.

Doing real-time communications via a geostationary satellite? You're doing it wrong. That's what Low Earth Orbit is for.

@TomMcW: Eh? I'm not saying "games are faster". I'm saying "games are incomparable". Games and drones are two different things. With games you need quick reactions, to duck from people, to shoot at fast-moving targets, and to generate a lot of high-res 3D imagery in real time. With a drone, sure, you don't need that high-res imagery (because you have a camera instead of CGI) but you're also not trying to snipe at people running across a rooftop. You're dropping bombs and launching guided missiles. It's a totally different paradigm, one in which latency is far less of a barrier.

@LightnessRacesinOrbit "your little first person shooter computer games in your bedroom.". Videogames are a great deal more applicable than you might appreciate. A flight simulator is just a video game after all. You even get a score! Formula 1 drivers from smaller teams (who cannot afford a simulator) use retail racing simulators to learn new tracks. Get your head out of your ass grandpa. Seriously ... http://www.smithsonianmag.com/smart-news/military-contractor-just-went-ahead-and-used-xbox-controller-their-new-giant-laser-cannon-180952647/?no-ist

@Gusdor: A seconds' lag in a flight simulator at cruise would be just fine. Either Tom was wrong or he was talking about a different kind of game, one which is more commonly played multiplayer with users complaining about lag: most likely a first-person shooter. And quit cursing/name-calling. Seriously ...

+1 to the op. You know it's a good question when it sparks a brawl in the comments! ...am I missing a tooth?

The aerodynamic surfaces of a drone are controlled by computers, not by humans. When a human makes a control input this input is broadcast to the UAV, and the computers on the UAV make the appropriate changes to the flight surfaces to carry that input out. It's not direct control of the drone by humans via radio.

Do you mean the drone has some level of autonomy and can react to events without the immediate intervention of the pilot? Maybe the pilot only send waypoints / targets, and the drone manages to go there?

I think what he's saying is that the drone operator will move the joystick, and that tells the plane "Bank right 10 degrees" and the avionics computer figures out how to do that.  Just like a fly-by-wire fighter jet would do.  It's not like an hobbyist R/C plane where moving the stick directly controls the flight surfaces.

@JPhi1618: That's what I was thinking too, just like entering commands on the MCP of a B737.

That's exactly what I mean @JPhi1618

"Do you mean the drone has some level of autonomy and can react to events without the immediate intervention of the pilot? " Isn't that the definition of Drone? Otherwise it would just be a big R/C plane.

Source: Acquaintances who worked on avionic software and hardware development

Latency matters for anything mechanical that flies !
On-board equipments are developed to handle sub-second interactions. Any interaction from ground is assumed to take many 100s of milliseconds to reach the flight (anything that flies). Humans operating the flight are trained to be quick and anticipate rather than react !
To minimize the impact of latency, a series of information keeps flowing thereby converting latency problem into throughput problem. Throughput problems can be handled using multiple beacons / transmitting sources. This does not reduce latency but pre-loads onboard systems with accurate data to anticipate next 500 mS or so.
To understand the solutions for latency, look for 2 businesses that are latency sensitive - gaming and high frequency trading. To put things in perspective, in HFT, a market event is detected by software and it generates an order and sends it - all under 1 micro-sec. Further latencies in network stack is not measured here but it gives you a sense of what operates here.

In the case of high-frequency trading, it seems that what's really going on is not that companies wish to react to e.g. the fact that a stock's price drops, but rather that they want to be able to have a limit buy order pending in such a fashion as to be invisible until such time as it could be fulfilled.  The same sort of semantics could be offered without need for instant communication if the market had a trustworthy entity which would pair up buyers and sellers without revealing any information about pending limit orders.  In any case, the key to efficiency is to express decisions...

...made on the basis of incomplete information, in a way that indicates in advance what should be done with information that becomes available but which wasn't available when the decision was made.

I think everything you said except about HFT. HFT avoids latency in the host, but they also do everything to shrink the transit time, whereas drones cannot really do that.

A significant number of these devices use "top/level" control. The operator may have a camera to see what is in front of the device but the main screen they are looking at is a top view of the landscape (including height map) of where it is flying. The altitude is set to x meters/yards above ground/sea level. The device maintains height and direction based on this input. As the drone isn't in live action and dodging anything - the delays and adjustments do not impact the route of the machine.
E.g. A 90 degree turn only requires 2 signals. One to initiate the turn and the other back to base to confirm the turn is complete and the drone's current positioning.
You would rarely find a full scale drone needing manual adjustment of anything less than 500 meters. Also the video footage from the drone is usually only uploaded at full quality once it has returned home.

Welcome. You may want to edit your question (using the edit link) and include your comment content and delete it.


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What does the runway distance mean under "Additional Runway Information" on Jeppesen plates?
See my previous question
What are Runway Declared Distances?
On Jeppesen 10-9 or 10-9A charts, there is a section dealing with Additional Runway Information.  Does these distances correspond to the declared distances in the A/FD?

From page AIRPORT-3 (page 3 overall) of this Jeppesen legend (site jump-off page here):